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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_functiontypes.about_tag_types"></a><a class="link" href="about_tag_types.html" title="About Tag Types">About Tag Types</a>
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<p>
      Boost.FunctionTypes uses tag types to encode properties that are not types
      per se, such as calling convention or whether a function is variadic or cv-
      qualified.
    </p>
<p>
      These tags can be used to determine whether one property of a type has a particular
      value.
    </p>
<pre class="programlisting"><span class="identifier">is_function</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">(...),</span> <span class="identifier">variadic</span><span class="special">&gt;::</span><span class="identifier">value</span> <span class="comment">// == true
</span><span class="identifier">is_function</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">()</span>   <span class="special">,</span> <span class="identifier">variadic</span><span class="special">&gt;::</span><span class="identifier">value</span> <span class="comment">// == false
</span></pre>
<p>
      A compound property tag describes a combination of possible values of different
      properties. The type <code class="literal">components&lt;F&gt;</code>, where <code class="literal">F</code>
      is a callable builtin type, is a compound property tag that describes <code class="literal">F</code>.
      The <code class="literal">tag</code> class template can be used to combine property tags.
    </p>
<pre class="programlisting"><span class="identifier">tag</span><span class="special">&lt;</span><span class="identifier">non_const</span><span class="special">,</span><span class="identifier">default_cc</span><span class="special">&gt;</span> <span class="comment">// combination of two properties
</span></pre>
<p>
      When several values for the same property are specified in <code class="literal">tag</code>'s
      argument list, only the rightmost one is used; others are ignored.
    </p>
<pre class="programlisting"><span class="identifier">tag</span><span class="special">&lt;</span><span class="identifier">components</span><span class="special">&lt;</span><span class="identifier">F</span><span class="special">&gt;,</span> <span class="identifier">default_cc</span><span class="special">&gt;</span> <span class="comment">// overrides F's calling convention property
</span></pre>
<p>
      When compound property tag is specified to analyse a type, all of its component
      properties must match.
    </p>
<pre class="programlisting"><span class="identifier">is_member_function_pointer</span><span class="special">&lt;</span> <span class="identifier">F</span><span class="special">,</span> <span class="identifier">tag</span><span class="special">&lt;</span><span class="identifier">const_qualified</span><span class="special">,</span><span class="identifier">default_cc</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">value</span>
<span class="comment">// true for 
</span><span class="comment">//   F = void(a_class::*)() const
</span><span class="comment">// false for
</span><span class="comment">//   F = void(a_class::*)()
</span><span class="comment">//   F = void(__fastcall a_class::*)() const
</span></pre>
<p>
      Default values are selected for properties not specified by the tag in the
      context of type synthesis.
    </p>
<pre class="programlisting"><span class="comment">// given S = mpl::vector&lt;int,a_class const &amp;&gt;
</span>
<span class="identifier">member_function_pointer</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="comment">// is int (a_class::*)() const
</span><span class="comment">// note: the cv-qualification is picked based on the class type,
</span><span class="comment">// a nonvariadic signature and the default calling convention 
</span><span class="comment">// are used
</span>
<span class="identifier">member_function_pointer</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">,</span><span class="identifier">non_const</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="comment">// is int (a_class::*)()
</span><span class="comment">// no const qualification, as explicitly specified by the tag type
</span></pre>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2004-2007 Tobias
      Schwinger<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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